Pleuromutilin derivatives, process for their preparation and uses thereof

A compound of formula (IA) or (IB):

• wherein:
• R1 is phenyl or a five- or six-membered heteroaryl ring, wherein R1 is substituted by a carboxylic acid group that is attached at a position which is not adjacent to the point of attachment of the sulphur, and wherein R1 is optionally further substituted by up to four groups independently selected from halogen, (C1-6)alkyl, aryl, aryl(C1-6)alkyl, (C1-6)alkoxy, (C1-6)alkoxy(C1-6)alkyl, halo(C1-6)alkyl, aryl(C1-6)alkoxy, hydroxy, nitro, cyano, azido, amino, mono- and di-N-(C1-6)alkylamino, acylamino, arylcarbonylamino, acyloxy, carbamoyl, mono- and di-N-(C1-6)alkylcarbamoyl, (C1-6)alkoxycarbonyl, aryloxycarbonyl, ureido, guanidino, (C1-6)alkylguanidino, amidino, (C1-6)alkylamidino, sulphonylamino, aminosulphonyl, (C1-6)alkylthio, (C1-6)alkylsulphinyl, (C1-6)alkylsulphonyl, heterocyclyl, heteroaryl, heterocyclyl(C1-6)alkyl and heteroaryl(C1-6)alkyl, or two adjacent ring carbon atoms may be linked by a (C3-5)alkylene chain, to form a carbocyclic ring;
• R2 is vinyl or ethyl;
• and
• R3 is hydrogen, hydroxy or fluorine and R4 is hydrogen,
• or R3 is hydrogen and R4 is fluorine;
• or a pharmaceutically acceptable derivative thereof